Wondering About major/minor 3rd use

Whenever I jam with someone and they say "lets do something in so and so key." Lets say A for example. I've heard a lot and even been told you HAVE to use the major third if you're playing in a major key. But I don't always do that. A lot of times I play the minor third because to my ears it sounds better. Don't know why, it just does. Yet..the small bar crowd still dances. And the rule I go by is if they're dancing, it's working!

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Yeah, I've noticed that too. It can sound really cool when switching around between the minor and major third (and sixth too).

It probably matters a lot what the other instruments are doing. It's a little trickier when the guitar or piano are playing chords that are emphasizing a different third than the one we're playing, eh?

It is usually wrong. For a pentatonic blues it'll work ok since the flatted third is a "blue note", but for most other things in the major key the flatted third is just wrong and playing one will collide and cause discordance with the other instruments (or singer) especially if they use a third.

The bar crowd is not always going to be theory-literate. Besides, they can dance even if the bass player is playing out of key.

So you're saying that if you're in A, you're playing A-C-E instead of A-C#-E? You can do it with a passing tone, but it can cause dissonance with the guitars/keys if you're not careful.

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Yes. I don't always do this but I do it often. I wont sit there and just straight play A-C-E. But If someone says A, I'll come up with a groove where I use the C note somewhere instead of the C# and If it sounds bad I'll change it really quick, but it hardly ever sounds bad. Also, it seems to work really well when a guitarist is playing funk style guitar like and E9 or A9, etc. I also do this a lot when I play blues bass lines. If someone says lets do a blues in E. I play out of the E minor pentatonic instead of the major or I'll throw them together chromatically. I've never gotten any complaints, yet haha.

If you play a minor 3rd, and guitarist or keys plays a Major 3rd, then it's going to be dissonant. However if the song goes from chord I to chord IV, then the minor 3rd as a passing note works pretty well, as long as the Major 3rd in the first chord isn't prominent.

Well it depends on whats going on in the song. Maybe you're playing the minor third during a bridge which isnt in Amajor? Either way you should probably pay more attention to each note, you should notice that it sounds out of place, especially an incorrect third. I have a hard time choosing the correct 7ths if I dont know the scale/key w.e The 3rd, IME, is much easier to identify. I know if I was playing a song in Amajor and someone kept using the minor3rd I'd get a bit frustrated...

Also, it seems to work really well when a guitarist is playing funk style guitar like and E9 or A9, etc. I also do this a lot when I play blues bass lines. If someone says lets do a blues in E. I play out of the E minor pentatonic instead of the major or I'll throw them together chromatically.

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The examples you've posted here suggest a blues or Mixolydian context - in other words, a Dominant 7th (or Dominant 9th (root, nat 3rd, 5th, b7th, 9th) chord is the tonic. Thus, you are not playing the b3rd, you are playing the #9, a common occurrence in these types of compositions.

Sure, the #9 and b3 are enharmonically equivalent to one another, but they serve entirely different functions - and this is why you are confused as to why this note sounds OK much of the time. The note c natural (or, more properly, B#) often sounds fine over an A7 chord, but not over an AMaj7 chord. So, the distinction is more than Major vs minor - which is another example of why I believe that chords should be explained not as triads, but as 1 3 5 7 stacks. In this case, the #9 works with a 1 3 5 b7, but not so well with a 1 3 5 7.